Recent advances in digital printing technology have made it possible to implement digital, i.e., bit map addressable, printing for the purpose of evidencing payment of postage by a postage-meter-like device. Where necessary in order to distinguish such postage-meter-like devices from the typical postage meter, such devices will be called herein Postage Evidencing Devices or PED's. In such devices, the printer may be a typical stand-alone printer. The computer driven printer of such a PED can print the postal indicia in a desired location on the face of a mail piece. Further, as used herein the postal indicia will be defined as the Postal Revenue Block or PRB. The PRB typically contains data such as the postage value, a unique PED identification number, the date and in some applications the name of the place where the mail is originating. It must be noted, however, that the term postage meter as used herein will be understood to cover the various types of postage accounting systems including such PED's and is not to be limited by the type of printer used.
From the Post Office's point of view, it will be appreciated that a serious problem associated with PED's is that the digital printing makes it fairly easy to counterfeit the PRB since any suitable computer and printer may be used to generate multiple images. In fact, many of these new PED systems may be using printers that are able to print legitimate indicias which are indistinguishable from those printed by others that are printed without any attempt to purchase postage.
In order to validate a mailpiece, that is to assure that accounting for the postage amount printed on a mailpiece has been properly done, it is known that one may include as a part of the franking an encrypted number such that, for instance, the value of the franking may be determined from the encryption to learn whether the value as printed on the mailpiece is correct. See, for example, U.S. Pat. Nos. 4,757,537 and 4,775,246 to Edelmann et al. as well as U.S. Pat. No. 4,649,266 to Eckert. It is also known to authenticate a mailpiece by including the address as a further part of the encryption as described in U.S. Pat. No. 4,725,718 to Sansone et al and U.S. Pat. No. 4,743,747 to Fougere et al.
U.S. Pat. No. 5,170,044 to Pastor describes a system wherein include a binary array and the actual arrays of pixels are scanned in order to identify the provider of the mailpiece and to recover other encrypted plaintext information. U.S. Pat. No. 5,142,577 to Pastor describes various alternatives to the DES encoding for encrypting a message and for comparing the decrypted postal information to the plaintext information on the mailpiece.
U.K. 2,251,210A to Gilham describes a meter that contains an electronic calendar to inhibit operation of the franking machine on a periodic basis to ensure that the user conveys accounting information to the postal authorities. U.S. Pat. No. 5,008,827 to Sansone et al. describes a system for updating rates and regulation parameters at each meter via a communication network between the meter and a data center. While the meter is on-line status registers in the meter are checked and an alarm condition raised if an anomaly is detected.
It will be appreciated that in order to verify the information in the PRB using the encrypted message, the verifier must first be able to obtain the key used by the particular meter. In trying to deal with mailing systems which may incorporate such encryption systems, it must be recognized that the meter population is large and subject to constant fluctuation as meters are added and removed from service. If the same key were to be used for all meters, the key distribution is simple but the system is not secure. Once the code is broken by anyone, the key may be made available to others using the system and the entire operation is compromised. However, if separate keys are used respectively for each meter then key management potentially becomes extremely difficult considering the fluctuations in such a large population.